1. Field of the Invention
The present invention relates to an image heating device that heats an image formed on a recording material which is preferably used as a fixing device of an image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
Up to now, as a heating device disposed in an image forming apparatus such as a copying machine, a printer or a facsimile machine using an electrophotographic system, there has been known an image heating fixing device that heats a toner image formed on a surface of a recording material in a direct manner or an indirect (transfer) manner by using toner (visualizing agent) made of a heating fusion resin through an appropriate image forming process means such as an electrophotographic, an electrostatic recording or a magnetic recording in an image forming portion of an image forming apparatus to heat and fix the toner image on the surface of the recording material as a permanently fixed image.
Up to now, as the image heating fixing device of the above type, there exists a roller heating device in which a recording material that bears a non-fixed toner image as a material to be heated is introduced into a pressure contact nip portion (fixing nip portion) that are a pair of rotary rollers made up of a fixing roller (heat roller) as a heat conductance rotary member which includes a heating means such as a halogen lamp therein and whose temperature is adjusted to a predetermined fixing temperature by the heating means, and a pressure roller that rotates in pressure contact with the fixing roller, and heats and pressurizes the recording material while nipping and transporting the recording material, to thereby heat and fix the non-fixed toner image on the recording material surface.
As usual, in the roller heating device, in order to keep the fixing roller to the predetermined temperature while controlling the lighting of the halogen lamp, a temperature detecting means such as a thermistor is brought in contact with the surface of the fixing roller to detect the temperature of the fixing roller surface. The temperature detecting means can be so arranged as to face a non-sheet passing region of the fixing roller (a region through which the recording material does not pass) or a sheet passing region of the fixing roller (a region through which the recording material passes).
However, the above-described conventional roller heating device suffers from the following problems in accordance with the detected position (a sheet passing region arrangement, a non-sheet passing region arrangement, or a non-image region arrangement) of the temperature detecting means on the fixing roller surface.
1) Non-sheet Passing Region Arrangement
The temperature detecting means that detects the temperature of the non-sheet passing region of the fixing roller is advantageous in that there occurs no stain caused by the abraded powder (paper dust) of the recording material per se, resulting in no occurrence of an image failure, because the temperature detecting means is abutted against the region through which no recording material passes. However, because it is necessary to estimate the temperature of the sheet passing region, it is difficult to conduct an accurate temperature control. In particular, in the case where a releasing layer or an elastic layer made of rubber or fluorine resin is disposed on the fixing roller, there is a case in which there occurs a drawback (hot offset) that a temperature difference between the non-sheet passing region and the sheet passing region becomes remarkable, and the temperature of the sheet passing region becomes high, to thereby contaminate the roller with the toner image, or a case in which there occurs a problem (fixing failure) that the temperature of the sheet passing region becomes low, thereby disenabling the fixing operation.
2) Sheet Passing Region Arrangement
The temperature detecting means that detects the temperature of the sheet passing region of the fixing roller can maintain a temperature proper for fixing even if rubber or the like is disposed on the fixing roller because the temperature detecting means detects the temperature of a portion through which the recording material passes. However, there is a case in which the strain caused by a slight amount of toner on the fixing roller surface is dammed and stored by repeating the printing operation, and the stored toner is sometimes discharged toward the fixing roller surface to cause the image strain (dropping). Also, there is a case in which the fixing roller surface is damaged by the temperature detecting means to make the image non-uniform.
In order to solve those problems, there is proposed a method in which a halogen heater having the same output is located within the pressure roller (a roller that is abutted against a surface of the recording material on the opposite side to the non-fixed toner image surface) so as to reflect the temperature of the fixing roller to control the lighting of the halogen heating at the fixing roller side by detecting the temperature of the pressure roller surface. However, because a power that can be supplied to the halogen heater is divided, there is a case in which the temperature of the fixing roller surface is deteriorated at a high speed device to cause the fixing failure during continuous usage.
In addition, there is proposed a method in which a thermopile, a thermistor or the like is made to face the fixing roller in a non-contact manner, to thereby prevent the toner strain and to measure the temperature of the sheet passing region. However, because the temperature detection precision of the non-contact thermistor is greatly affected by the temperature of the non-contact thermistor per se, very complicated algorithm and a large number of detection patterns must be prepared in order to accurately detect the wide temperature region pertaining to the fixing roller of from the room temperature to the fixing temperature at various atmospheric temperatures, and this arrangement is not put in practical use.
3) Non-image Region Arrangement (Non-image Region Within the Sheet Passing Region)
There is an advantage that the problems with the above arranging methods 1) and 2) can be reduced. However, it is difficult to detect the temperature of the sheet passing region over the entire sheet width, and it is impossible to prevent the stain such as the paper dust. Therefore, the arrangement includes the problems of both of the above arrangements, and is not a fundamental solving means.
The present invention has been made under the above circumstances, and therefore an object of the present invention is to provide an image heating device which is capable of disposing a temperature detecting element within a sheet passing region.
Another object of the present invention is to provide an image heating device in which the temperature detecting element is not stained with toner or paper dust.
Still another object of the present invention is to provide an image heating device having a non-contact type temperature detecting element and a contact type temperature detecting element.
Yet still another object of the present invention will become apparent from the following description.